1. Field of Invention
The invention generally relates to photovoltaic solar cells and, more particularly, to high-efficiency photovoltaic solar cells based on III-V semiconductor compounds.
2. Description of Related Art
Solar cells based on III-V semiconductor compounds have demonstrated high efficiencies for the generation of electricity from solar radiation. Known single junction solar cells formed primarily of III-V materials have reached efficiencies up to 28.3% at 1 sun solar intensity. Known multijunction III-V solar cells have reached 36.9% efficiency at 1 sun and 43.5% under concentrated sunlight equivalent to several hundred suns (see M. A. Green et al., Progress in Photovoltaics: Research and Applications 20 (2012) 12-20). Such efficiency and power achievements make it possible to apply this technology to the space and terrestrial energy markets. The solar cells with the highest efficiencies to date have employed three subcells having different energy band gaps and arranged to permit each subcell to absorb a different part of the solar spectrum.
Each subcell of a known multijunction solar cell of interest comprises several associated layers, typically including a window, emitter, base, and back surface field (BSF). These terms are known to those skilled in the art and do not need further definition here. Each of the foregoing layers may itself include one or more sublayers. The window and emitter are of one doping polarity (e.g., n-type) and the base and back surface field are of the opposite polarity (e.g., p-type), with a p-n or n-p junction formed between the base and the emitter. If an intrinsic region exists between the doped regions of the emitter and the base, then it may be considered a p-i-n or n-i-p junction, as is well known to those skilled in the art. Those skilled in the art will also recognize that subcells may also be constructed without all of the foregoing layers. For example, subcells may be constructed without a window or without a back surface field. The subcell is typically designed such that the majority of the light absorption occurs in the emitter and base layers, which ideally have high quantum efficiencies.
When reference is made to the stacking order of subcells from top to bottom, the top subcell is defined to be the subcell closest to the light source during operation of the solar cell, and the bottom subcell is defined to be the subcell furthest from the light source. Relative terms like “above,” “below,” “upper,” and “lower” also refer to position in the stack with respect to the light source. A solar cell with only one subcell is known as a single junction solar cell. When there is more than one subcell, such subcells are typically connected in series by tunnel junctions with contacts made to the top and bottom of the stack. However, other contact configurations are possible.
In the prior art, the emitter and the base of a given subcell are either formed of the same material (e.g., GaAs), or the emitter is formed of a material with a higher band gap than that of the base. In the former case, the emitter-base junction is a homojunction, while in the latter case it is a heterojunction. Where heterojunctions are used in the prior art, the teaching is typically that having an emitter with a higher band gap than the base improves efficiency. An example of the prior heterojunction design is given in U.S. Pat. No. 5,316,593 to Olson et al. and U.S. Pat. No. 5,342,453 to Olson, where the emitter is AlInGaP with a band gap of at least 1.8 eV, and the base is GaAs with a band gap of 1.42 eV. Another example is in U.S. Pat. No. 7,071,407 to Fatemi et al., where the emitter of the middle subcell is formed of InGaP, with a band gap between 1.8-1.9 eV, while the base is InGaAs, with a band gap of 1.4 eV. In U.S. Patent Publication US2009/0078310, Stan and Cornfeld describe a middle subcell with an InGaP emitter and (In)GaAs base, and a bottom subcell with an InGaP emitter and an InGaAs base, wherein the InGaP emitters have higher band gaps than the (In)GaAs base layers. It is reported that use of a higher band gap material for the emitter can improve efficiency compared to a homojunction when the emitter of the homojunction is of poor quality. With a higher band gap emitter, the dark current is reduced and the voltage produced by the solar cell can be increased. Further, the fraction of incident light absorbed in the emitter is reduced when the band gap is increased. The base typically has a substantially higher minority carrier diffusion length than the emitter, and the overall quantum efficiency, and therefore current, of the solar cell can be improved by transferring light absorption from the emitter to the base. Fatemi et al. list additional advantages of using a heterojunction with high band gap emitter, including better lattice matching and increased transmission of light to lower subcells. Thus, where a heterojunction is used for an emitter-base junction in a solar cell in the prior art, the emitter has a higher band gap than the base, and the teaching is that this improves the voltage and current, primarily by decreasing the dark current in the emitter and increasing the transmission of light through the emitter.
Solar cell efficiency is the product of the short-circuit current, open circuit voltage and fill factor of the solar cell. One of the major factors influencing the fill factor is the series resistance. The contributions to the series resistance within a solar cell include the resistance of the metal grid, the resistance between the metal contacts and their adjacent semiconductor layers (i.e., “the contact resistance”), the lateral sheet resistance of the emitter and/or window layers of the top subcell, the resistance of the tunnel junction(s) in the solar cell (if present), and the vertical resistance due to the resistivity of the individual solar cell and buffer layers and substrate. Additional non-negligible contributions to the series resistance may be produced by the wiring and other external elements in the solar cell circuit. In addition to reducing the solar cell fill factor, the solar cell current can be indirectly reduced by higher lateral sheet resistance values because the optimization of the grid design will lead to larger fractions of the solar cell covered by the metal grid, and shadowing the cell. Because resistance losses scale with the square of the current (i.e., “I2R”), they become increasingly important with larger values of current through the solar cell, such as due to increasing concentrations of light on the solar cell.
The lateral sheet resistance through the emitter and/or window layers of the solar cell is the resistance for the current to travel laterally through the emitter and/or window layers toward the metal grid, where the current is collected. Typically, when both an emitter and window are present, the emitter has a significantly lower sheet resistance than the window and most lateral current flow is through the emitter. For this reason and for simplicity, this lateral resistance will be referred to below as the “emitter sheet resistance.” Discussion will focus primarily on the resistivity of the emitter, even though the window may also contribute to this resistance value. The emitter sheet resistance is an important contributor to the total series resistance of the solar cell. As mentioned above, it influences the fill factor, especially at high concentrations, as well as the grid design. When the emitter sheet resistance is included in the design of the metal grid, including the grid width and spacing, higher sheet resistance values lead to more coverage of the solar cell by the grid (i.e., “grid shadowing”) in order to maximize the efficiency; this in turn reduces the solar cell current. There is a balance between grid shadowing and resistance losses that can be optimized for a given current flow through the solar cell. Particularly at concentrations of light equivalent to hundreds of suns or more, the emitter sheet resistance can have a significant impact on the overall solar cell efficiency.
Designs for three junction solar cells are rapidly approaching their practical efficiency limits. In order to reach even higher efficiencies, a greater number of subcells are evidently needed. In designing solar cells with more than three junctions, it is often advantageous for the top subcell to be higher in band gap than for a related three junction solar cell. For example, the conventional lattice-matched three junction solar cell on Ge has an InGaP top subcell, InGaAs middle subcell and Ge bottom subcell. In a related four junction, lattice-matched design on Ge, the top subcell is ideally formed of AlInGaP, with the addition of Al providing for a higher band gap than that of InGaP. However, because carrier mobility typically decreases as band gap increases, the emitter sheet resistance increases with band gap, when majority carrier concentration is held constant. Further, the maximum majority carrier concentrations that can be obtained by extrinsic doping also tend to decrease with increasing band gap, which can contribute to additional increases in the emitter sheet resistance. For example, Z. Z. Sun et al. (Journal of Crystal Growth 235 (2002) 8-14) found that the electron and hole mobilities of (Al0.7Ga0.3)0.52In0.48P are between factors of 2 times and 10 times lower than those of the lower band gap Ga0.52In0.48P for the same majority electron and hole concentrations. They also reported that the maximum electron and hole concentrations achieved by doping with Si and Be, respectively, are lower for (Al0.7Ga0.3)0.52In0.48P than Ga0.52In0.48P. Thus, due to the decreases in both carrier concentration and mobility, the emitter sheet resistance will typically increase as the band gap of the top subcell increases. While it is nearly always advantageous to reduce the emitter sheet resistance, these increases provide further motivation for reducing emitter sheet resistance as top subcell band gap increases.
Efforts to reduce emitter sheet resistance are also becoming increasingly important for solar cells used in concentrating photovoltaic applications, where there is growing interest in using higher concentrations of sunlight that produce higher solar cell currents and therefore higher resistance losses. While employing greater numbers of subcells does typically reduce the overall solar cell current, reducing the series resistance losses, the overall efficiency can still be improved by minimizing the emitter sheet resistance. However, typical strategies for reducing emitter sheet resistance involve corresponding losses in short-circuit current, such as increasing the emitter doping level, which decreases the minority carrier diffusion length. Significant efficiency gains will only be realized when the emitter sheet resistance is decreased without reducing the short-circuit current.
In addition to the motivation to find structures with reduced emitter sheet resistance, there are a number of instances when generating additional current in one or more subcells of a multijunction solar cell, with minimal impact on the open-circuit voltage, can be advantageous. For example, in a known InGaP/InGaAs/Ge triple junction solar cell, the bottom Ge subcell has the capacity to produce more short-circuit current than the upper subcells. Structures can be found in the prior art that increase the short-circuit current of the upper subcells using quantum wells, quantum dots, or adding a lower band gap region at the back of the base of the middle junction. However, the bottom Ge cell still produces excess current, and so there is motivation to find additional methods for increasing the current of the upper subcells. In addition, for solar cells used in outer space, radiation damage can affect the short-circuit current of one subcell more than the others, making it advantageous for this subcell to have higher short-circuit current values before radiation exposure.